Role of steam vapor explosions during core meltdown of LWR's

Steam vapor explosions (FCI's), possibly occurring during a severe core meltdown of a LWR are assessed. Important input parameters like amount, composition and physical data of the melt before the interaction are given and discussed on behalf of computer program calculations. The failure modes of the core supporting structure are discussed together with the behavior of the melt in the collapse- and premixing phase for two cases: dry and water covered core at the beginning of the accident. In the latter case, chances for a severe FCI are estimated to be higher. The premixing and fuel-coolant interaction phase is investigated also; estimations show, that large scale premixing of fuel and melt, necessary for a severe, coherent FCI, is possible only under very improbable circumstances. Assuming a large scale FCI, effects of shock waves and rapid gross steam production are discussed